What is Brain Stimulation?
Brain stimulation involves activating or inhibiting certain parts of the brain using electricity or magnetic fields. Apart from being used in clinical research to improve the understanding of the human brain, different brain stimulation therapies have shown effectivess in the treatment of many diseases.
Brain stimulation methods are usually divided into non-invasive and invasive methods. Non-invasive brain stimulation modulates brain excitability without the need for invasive surgery, while invasive brain stimulation methods require incisions or surgical procedures to directly interact with the brain.
Non-Invasive Brain Stimulation
Transcranial Direct Current Stimulation (tDCS)
Transcranial direct current stimulation (tDCS) is a neurophysiological technique for the effective, safe and non-invasive brain stimulation. It involves delivering low electrical currents (usually up to 2 mA) to the scalp via at least two electrodes. Depending on the polarity of the current at the target electrode, the patient can either an anodal or a cathodal stimulation, which, usually lead to an increase or decrease of the cortical excitability respectively (tDCS Database: http://tdcsdatabase.com/).
Already a short stimulation duration (~10 min) can have positive effects. These effects are due to electrophysiological changes in the neuronal cell membrane. Repeated tDCS over a prolonged period can lead to behavior effects, which might persist for months after the stimulation, i.e. pain reduction in chronic pain or improvement in cognitive ability in cognitively impaired patients.
Repeated anodal tDCS over the dorsolateral prefrontal cortex in unipolar depression have received level A (definitely effective) classification.
Transcranial Alternating Current Stimulation (tACS)
The transcranial alternating current stimulation (tACS) differs from tDCS as it applies a low-intensity sinusoidal electrical current to the brain. This occurs at a chosen frequency to interact with the brain’s cortical oscillations. This technique modulates brain function by entraining brain oscillations and inducing long-term synaptic plasticity, potentially impacting cognitive processes.
tACS has been studied in cognitive neuroscience and is now being explored in psychiatric clinical trials as a potential therapeutic tool. The stimulation parameters, such as frequency, amplitude, and duration, can be customized to target specific brain regions. By altering brain oscillations and connectivity patterns, tACS may offer a way to modify neuronal activity without directly affecting firing rates which can change the excitability or activity in specific brain areas.
Transcranial Random Noise Stimulation (tRNS)
Transcranial random noise stimulation (tRNS) employs low-intensity alternating currents, which are randomized in terms of their frequency and amplitude. This technique allows for the enhancement of neural excitability. Unlike other forms of transcranial electrical stimulation, tRNS introduces noise to the system, potentially enhancing neuronal sensitivity to weak inputs. The underlying mechanisms are yet to be understood.
This method has shown promising effects on cortical circuits, modulating neural processing in sensory and motor areas, and improving cognitive and motor performance. Explanations for the efficacy of the tRNS approach are based on the interference with cortical rhythms, plasticity changes or the improvement of the signal-to-noise ratio (SNR) which can lead to improved sensibility.
Transcranial Magnetic Stimulation (TMS)
Transcranial magnetic stimulation (TMS) utilizes strong magnetic fields to stimulate the brain non-invasively. The magnetic field induces an electrical current in the brain, which can increase or decrease the cortical excitability.
In contrast to the transcranial electrical stimulation, TMS can, depending on the intensity of the stimulation directly trigger action potentials, or inhibit them by creating a silent period.
TMS can also be used as a diagnostic tool on the motor cortex to investigate motor evoked potentials (MEP). MEP can serve as an indication for brain or spinal cord injuries (e.g. multiple sclerosis).
Repetitive TMS (rTMS) is used as a therapeutic tool to cause persistent changes in the activity of the cortex. This allows for the treatment of various diseases, e.g. depression, obsessive-compulsive disorder, migraines, among others.
Transcranial Ultrasound Stimulation (TUS)
Transcranial Ultrasound Stimulation (TUS) is a novel non-invasive technique that modulates neural circuits in the intact human brain by delivering pulsed ultrasonic waves to targeted brain regions. This method allows for the excitation or inhibition of neural activity, offering high spatial resolution and deep penetration capabilities.
TUS has shown promise in modulating specific neuronal pathways and nuclei, making it a valuable tool for both basic and clinical neurosciences. It may have therapeutic potential in treating neurological disorders such as Alzheimer's disease, epilepsy, stroke, and psychiatric conditions. TUS involves generating an acoustic wave through piezoelectricity, where electrical current induces vibrations in a crystal that release energy through an ultrasound transducer.
Electroconvulsivetherapy (ECT)
Electroconvulsive Therapy (ECT) is a medical procedure where small electric currents are passed through the brain under general anesthesia, intentionally triggering a brief seizure. ECT is used to treat severe mental health conditions such as severe depression, treatment-resistant depression, severe mania, and other mental illnesses. It can provide rapid and significant improvements in symptoms when other treatments have not been successful.
ECT works by causing changes in brain chemistry that can quickly reverse certain mental health conditions. Modern ECT is much safer than early treatments, with controlled electric currents administered in a controlled setting to minimize risks. The procedure typically involves a series of treatments over several weeks, with improvements often noticed after about six treatments. ECT may cause side effects like memory loss and confusion, but these are usually temporary. ECT is recognized as an effective treatment by various medical organizations and can be used across different age groups for severe mental health conditions.
Invasive Brain Stimulation
Deep Brain Stimulation (DBS)
Deep Brain Stimulation (DBS) is a neurosurgical procedure that involves implanting electrodes within specific areas of the brain. These electrodes produce electrical impulses that modulate brain activity to treat various medical conditions. DBS is commonly used to treat conditions such as Parkinson's disease, essential tremor, dystonia, epilepsy, Tourette syndrome, and obsessive-compulsive disorder. The electrical impulses can affect cells and chemicals in the brain related to these conditions. DBS is controlled by a pacemaker-like device under the skin, with wires connecting it to the electrodes in the brain. This technique is being researched for potential applications in treating conditions like Huntington's disease, chronic pain, cluster headaches, dementia, depression, addiction, and obesity.
Vagus Nerve Stimulation (VNS)
Vagus Nerve Stimulation (VNS) is a medical procedure that involves using an implanted device to send electrical pulses to the vagus nerve, which then transmits these impulses to the brain. This stimulation can modulate brain activity and is used to treat conditions such as epilepsy, depression, and as a rehabilitation aid for stroke patients. VNS is considered a type of neuromodulation that alters nerve activity. The procedure typically involves implanting a device under the skin in the chest area, with a wire lead attached to the vagus nerve in the neck.